The present invention relates to phase shifters, and in particular, to phase shifters useable in systems requiring a steerable antenna or an antenna that transmits or receives a modulated signal.
Conventional phase shifters include several classes of devices:
1) Tunable frequency selective surfaces —Tunable frequency selective surfaces provide varying transmission amplitude when the surface is tuned. These structures can also provide a phase shift when they are tuned. However, since this is accompanied by a change in amplitude, they are less useful since it is desirable to have the phase shifter transmit nearly constant amplitude while changing the phase.2) Tunable impedance surfaces —Tunable impedance surfaces provide a phase shift on reflection. However, reflective phase shifters are problematic for many applications where the platform cannot permit the geometry required for an external feed which would be required for a reflective phase shifter.3) Quasi-optical devices —Quasi optical devices typically provide amplification to a signal passing through them, but could also be designed to provide a phase shift. However, such amplification involves active devices needed for amplification, adding more system complexity.
Each class of conventional phase shifters has further difficulties. For example, the tunable frequency selective surface typically occupies a single layer, with that layer creating a frequency-dependent amplitude variation on the wave passing through it. Multi-layer frequency selective surfaces have been studied, but each layer has the same filtering effect and it would be problematic as to how to build up a multi-layer structure where each layer produces a progressive phase shift. For the tunable impedance surfaces, it would be problematic as to how to convert a reflective phase shifter into a transmissive phase shifter. For the quasi-optical devices, these typically also occupy a single layer, so the same problems as for the frequency selective surfaces applies here.